U.S. patent number 3,676,608 [Application Number 05/082,272] was granted by the patent office on 1972-07-11 for line circuit for a key telephone system.
This patent grant is currently assigned to Bell Telephone Laboratories, Incorporated. Invention is credited to David Clayton Goldthorp, Albert David Limifro, John Paul Smith.
United States Patent |
3,676,608 |
Goldthorp , et al. |
July 11, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
LINE CIRCUIT FOR A KEY TELEPHONE SYSTEM
Abstract
In a key telephone system line circuit employing relays and
transistor control and logic circuitry, elimination of the
conventional central office ground return path is achieved by
substantially isolating a bridged ringing detector relay from the
logic and control elements. Breakdown voltage limitations of
integrated circuitry are met by the employment of a stacking
arrangement for certain of the transistors of the control circuit
that are utilized to drive one of the control relays.
Inventors: |
Goldthorp; David Clayton (Oley,
PA), Limifro; Albert David (Denver, CO), Smith; John
Paul (Broomfield, CO) |
Assignee: |
Bell Telephone Laboratories,
Incorporated (Murray Hill, Berkeley Heights, NJ)
|
Family
ID: |
22170153 |
Appl.
No.: |
05/082,272 |
Filed: |
October 20, 1970 |
Current U.S.
Class: |
379/166 |
Current CPC
Class: |
H04M
9/006 (20130101) |
Current International
Class: |
H04M
9/00 (20060101); H04m 001/00 () |
Field of
Search: |
;179/81R,84R,84A,9K,99 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Helvestine; William A.
Claims
What is claimed is:
1. A telephone line circuit for a key telephone system comprising,
in combination,
a pair of line conductors, ringing detector means bridged across
said conductors,
a transistor network responsive to said detector means for
initiating the operation of local ringing means,
means including a timing circuit capacitor for protecting said
network against operation by spurious signals thereby eliminating
the need for a ringing ground return path,
said transistor network being isolated from said ringing detector
means,
a plurality of relays for performing logic and output
functions,
said network including an interconnected pair of output transistors
for driving one of said relays,
one of said pair having the emitter electrode thereof connected
directly to the collector electrode of the other of said pair
and means connecting the collector electrode of said one of said
pair to said last named relay.
2. Apparatus in accordance with claim 1 including means for
ensuring the availability of the combination of the BVCES of said
other of said pair and the BVCEO of said one of said pair for
driving said last named relay.
3. Apparatus in accordance with claim 2 wherein said ensuring means
includes a junction point from which operating current is applied
to the base electrodes of said pair,
means including a first diode connecting said point to the base of
said one of said pair,
means including second and third diodes connecting said point to
the base electrode of the other of said pair,
a third transistor having the collector-emitter path thereof in
parallel with the base-emitter path of said other of said pair
and means directly connecting the collector electrode of said third
transistor to the base electrode of said other of said pair.
4. Apparatus in accordance with claim 3 including a local power
source for biasing the transistor of said network and for operating
said relays,
and dual function means comprising a single transistor connected in
common emitter configuration between said source and said network
for operating as a current limiter and voltage stabilizer.
5. Apparatus in accordance with claim 4 wherein all the transistors
of said network including said pair and said single transistor are
included in a single integrated circuit.
6. A telephone line circuit for a key telephone system comprising,
in combination, a pair of conductors connectable to a telephone
line, a bridged ringing detector including a ringing-responsive
relay connected across said pair of conductors, means including a
transistor network, a timing circuit and a plurality of relays
responsive within preselected parameters to the operation of said
ringing-responsive relay for effecting operation of said plurality
of relays thereby to initiate local signaling response, said
bridged ringing detector being isolated from said means with the
exception of contacts of said ringing-detector relay being included
as a part of said timing circuit, said said network including first
and second interconnected transistors arranged as a relay driving
circuit for one of said plurality of relays, the emitter electrode
of said first transistor being connected to the collector electrode
of said second transistor, the collector electrode of said first
transistor being connected to said last named relay and means for
ensuring the availability of the combination of the BVCEO of said
first transistor and the BVCES of said second transistor for
driving said one relay.
7. Apparatus in accordance with claim 6 wherein said ensuring means
comprises a third transistor having the collector-emitter path
thereof connected across the base emitter path of said second
transistor,
said third transistor being in a saturated state when said one
relay is to be operated.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to telephone line circuits and more
particularly to line circuits of the type utilized in key telephone
systems.
2. Description of the Prior Art
A telephone line circuit forms an essential part of a telephone
subscriber's equipment in that it provides an interface between a
subscriber's telephone set proper and the corresponding central
office, PBX or other switching point. The specific functions
performed by a telephone line circuit involve those aspects of
supervision and control that are incident to the establishment of a
connection between a control switching point and one or more local
subscribers's telephone sets. Such functions typically include
signaling a particular subscriber's station in response to the
detection of ringing current on the line. In some arrangements the
ringing signal from the switching point may be utilized directly to
operate a local ringer at the subscriber's station. In other
arrangements the received ringing signal may be detected by the
line circuit which, in turn, extends ringing current from a local
source to an individual ringer or ringers. The latter arrangement
may be employed, for example, in key telephone systems where there
is a need for signaling a number of subsets in response to an
incoming call.
In conventional circuits it is usual to provide for a ground return
path from the line circuit ringing detector to the connecting
central office. This path, as shown for example by R. E. Barbato
and D. T. Davis in U.S. Pat. No. 3,436,488, issued Apr. 1, 1969, is
established by extending a center tap from a resistive-capacitive
bridge across the line to that portion of the circuit which
includes a ground point and elements of the control and logic
circuitry. The employment of a ringing ground return path of this
type is satisfactory under favorable ground conditions but problems
arise in those areas where ground impedances are high and uneven.
Under such circumstances unbalanced potentials are created which
may result in the detection of spurious ringing signals.
Elimination of this problem is one of the objects of this
invention.
A primary reason for the use of a ground return path for ringing
current in the prior art is the need to protect the ringing
detector circuits of individual stations in a key telephone system
from being triggered falsely by dialing pulses applied to the
telephone line by another one of the stations of the system that is
connected only to the telephone line. Broadly, this protection is
achieved by a timing circuit arrangement that is connected directly
to the center tap of the ground return circuit. Although dial pulse
interference is largely absorbed by this method, it has the
inherent disadvantage of exposing the ringing detector circuit to
other forms of interference. Avoidance of this disadvantage is
another of the objects of this invention.
Another problem known to occur in prior art line circuits is that
fluctuations in line circuit operating current occur with
variations in loop length. Stabilization of line circuit operating
current without undue cost or complexity is thus another object of
the invention.
A further problem relates to the fact that prior art line circuits
have not exploited the advantages of high reliability and low cost
that are inherent in the use of integrated circuits. The continued
use of discrete components has resulted in part from the difficulty
of fabricating integrated transistors with desirable properties
which also will withstand the voltages required for driving relays.
An additional object of the invention, therefore, is to employ
integrated circuitry in a telephone line circuit without
sacrificing the otherwise desirable properties of relatively low
voltage output transistors.
SUMMARY OF THE INVENTION
The foregoing objects and additional objects are achieved in
accordance with the principles of the invention by a line circuit
that affords complete protection against dial pulse interference
and other forms of interference without being subjected to the
disadvantages of a ground return path for ringing current. One of
the key features of the invention involves the elimination of the
conventional ground return path and the substantial isolation of
the bridged ringing detector relay circuit from the elements of the
detection and logic circuit. This arrangement is made possible in
part by the substitution of local power for power conventionally
derived from line current. As a result, the buffer or timer
capacitor has one side connected to the local power source instead
of to the potential interference sources of the line.
Another feature of the invention involves a unique means for
limiting the current supplied to the detection and logic circuit
and for stabilizing the voltage from the local current source. Both
functions are performed by a single transistor which is a part of
an integrated circuit which combines all the circuit elements,
aside from relays, required for ringing detection and logic.
Still another feature of the invention relates to a stacking
arrangement of relay-driving output transistors that provides
sufficient voltage to effect relay operation without, however,
exceeding the relatively low voltage limits on transistors that are
otherwise uniquely suited for fabrication by integrated circuit
techniques.
BRIEF DESCRIPTION OF THE DRAWING
The single FIGURE is a schematic circuit diagram of a line circuit
in accordance with the invention in combination with a typical key
telephone set and a connecting interface or panel circuit.
DETAILED DESCRIPTION
The line circuit 101 in the drawing includes a bridged ringing
detector 101A connected across the telephone line RT, and a logic
and control circuit 101B which, in accordance with the invention,
is isolated from the ringing detector portion 101A.
The ringing detector 101A includes a multifunction line relay L in
combination with associated relay contacts and circuit elements.
The logic and control circuit 101B employs transistors Q1-Q4 and
their associated circuit components to effect various logic,
detection and relay control functions. Transistors Q5, Q6 and Q8
together form a separate relay drive circuit for relay B, and
transistor Q7 operates as a combination current limiter and voltage
control device. The specific functions of relays A, B and C of the
logic control circuit 101B are best presented in terms of the
operational description that is presented hereinbelow.
A panel circuit 102 provides various power and signaling
connections as well as interface connections for a telephone 103.
The only elements of the telephone 103 that are illustrated are the
pickup key contacts PU.sub.1, PU.sub.2, a substation network 104,
switch hook contacts SW.sub.1 and SW.sub.2 and a signal light
LT.
It will be noted that in the bridged ringing detector 101A relay L
is shunt connected and is in circuit combination with contacts from
the A, B and C relays, along with resistors R1 and R2 and a
capacitor C1. In accordance with the invention, there are no
conducting paths which lead from either the ring or the tip side of
the telephone line to the logic or control circuitry found below.
This arrangement is in contrast to prior art arrangements as shown,
for example, in the Barbato-Davis patent cited above. Elimination
of the ringing current ground return path, in accordance with the
invention, effects reduction in the number of circuit components
required, with an enhancement in reliability and a reduction in
cost, together with the advantages of avoiding the occasional
dilution of service quality, which is typical when ground return
paths are used in areas of poor grounding characteristics.
INCOMING SIGNAL
In the idle circuit condition, each of the relays A, B and C is in
the unoperated state, and transistors Q3, Q4, Q5, Q6 and Q8 are
off. Transistors Q1 and Q2 are held on by current supplied to the
base of transistor Q1 through the resistors RT1, RT2 and R4.
Ringing voltage is applied across the line with either side of the
line grounded. As indicated above, a bridged ringing detector is
used which does not require a ground return circuit or path to the
detecting central office of PBX. Ringing current flows through the
series connected primary and secondary windings of relay L,
resistor R2 and capacitor C1 causing relay L to operate on each
half cycle of ringing current. The base of transistor Q1 is
connected to a negative power supply P1, which may be 24 volts for
example, through resistor R4, relay contact L.sub.1, resistor R17
and diode CR10. By the action of capacitor C3, however, transistor
Q1 is delayed from turning off for some preselected period such as
150 milliseconds.
When transistors Q1 and Q2 do turn off, the collector voltage of
transistor Q2 rises. This voltage change applied to the base of
transistor Q3 by way of resistor R5 turns transistor Q3 on, and the
voltage change effected thereby at the emitter of transistor Q3
causes the breakdown of diode CR3 which effects the turnon of
transistors Q4, Q5 and Q6. The combined output across transistors
Q5 and Q6 is employed, in accordance with the invention, in order
to operate relay B. During the turnoff process, the base-emitter
voltage of transistors Q5 and Q6 is pulled to zero through the
resistors R13 and R12. The collector of transistor Q6 thus rises in
potential to approximately one-half the supply voltage of power
supply P1 and the emitter and base of transistor Q5, in effect,
float up to the collector potential of transistor Q6. This
floating-up of potential at the base-emitter of transistor Q5 is
made possible primarily by the action of diode CR5.
Ignoring for the moment the function played by transistor Q8, the
stacked arrangement of transistors Q5 and Q6 permits turning off
inductive loads (such as a relay for example) with a supply voltage
equal to the sum of their collector-emitter breakdown voltages,
assuming open base electrodes (BVCEO), which in one embodiment of
the invention is 30 volts. With transistor Q8 in the circuit,
however, the effect is that of utilizing the collector-emitter
breakdown characteristic of transistor Q6 with its base shorted
(BVCES) rather than open. Transistor Q8 is in a saturated state
when transistors Q5 and Q6 turn off and as a result of the
combination of BVCES of transistor Q6 with the BVCEO of transistor
Q5, the circuit in the embodiment indicated is capable of switching
inductive loads up to 45 volts rather than 30 volts.
In accordance with the invention, diodes CR5, CR6 and CR7 together
with transistors Q5 and Q6 are, in a sense, employed as a form of
feedback circuit so that current applied to the CR5-CR6 junction
point is split in such a way that transistor Q6 is turned on to the
point where the collector voltage is approximately the same as the
Base Voltage of Q6. The other portion of the current is applied to
the base of transistor Q5 by way of diode CR5.
The particular stacking arrangement in which transistors Q5 and Q6
are connected was selected in accordance with the invention to
ensure the circuit capability of energizing and deenergizing relay
B without exceeding the voltage limitations of these transistors.
These limitations are imposed primarily by integrated circuit
parameters since all elements shown within the block IC are
combined in a single integrated circuit.
The base-emitter junction of transistor Q5 has about the same drop
as diode CR7 so that the collector of transistor Q6 reaches the
base potential of transistor Q6 in the on state. In the off state
the common node of diodes CR5 and CR6 is, in effect, open
circuited.
Relay C does not operate at the same time as relay B inasmuch as a
resistor R3 limits the current through its winding to less than its
operating value. Relay B operated connects ground to the ST lead by
way of make contact B.sub.3 and connects the L lead to the LF lead
by way of contact B.sub.2 to initiate flashing lamp signals and
interrupted ringing current. The RC lead is connected to ringing
circuits (not shown) through contact B.sub.5 for audible signal
control. Transistors Q1 and Q2 remain off, and transistors Q3, Q4,
Q5 and Q6 remain on until the call is answered or the circuit times
out.
TIME-OUT OF RINGUP CIRCUIT
When relay B operates, the negative end of a charged capacitor CT
is connected through contact B.sub.1 to the resistor RT1 and
through resistor R4 to the base of transistor Q1, holding
transistor Q1 off. During the interval that ringing is not present,
capacitor CT discharges to ground through resistors RT1 and RT2,
driving the base of transistor Q1 in a direction to turn transistor
Q1 on. If the L relay operates again before transistor Q1 turns on,
capacitor CT charges again through the closed B.sub.1 contact, the
L.sub.2 contact and resistor R17. If the L relay does not operate
again, capacitor CT discharges to a point sufficient to turn on
transistor Q1. The time it takes capacitor CT to discharge may be
on the order of twenty seconds.
A shorter time-out period may be provided in a manner similar to
that described above, with the exception that resistor RT2 may be
short circuited with an optional connection, not shown, thereby
lowering the resistance through which capacitor CT discharges. The
time-out period can be reduced still further by shunting the RT1
resistor with an optional connection, not shown. Adding resistance
across resistor RT1 also reduces delayed hold release time which is
controlled by capacitor C2.
ANSWERING AN INCOMING CALL -- BUSY STATE
An incoming call is answered by operating the pickup key associated
with the line being rung and going off-hook. The station shunt is
then connected across the line through the switch hook contacts
SW.sub.1 and key contacts PU.sub.1 and PU.sub.2 , and ringing is
tripped at the central office. Ground is also connected through the
switch hook contacts SW.sub.2 and the pickup key contacts PU.sub.3
to the A lead, causing relay A to operate. Relay A operated shunts
the primary or upper terminals of relay L by way of the contact
A.sub.2 thus preventing relay L from operating on dc line current.
The base of transistor Q1 is connected to the common side of the
voltage regulator through resistor R4 make contact A.sub.3 , and
resistor R15. Additionally, the power supply P1 is connected by way
of the diode CR10 and contact A.sub.4 to the winding of relay C,
causing it to operate. Relay C operated removes capacitor C3 from
the base of transistor Q1 and connects resistor R16 by way of
contacts C.sub.5 across capacitor C3, causing it to discharge.
Relay C operated also disconnects the negative end of capacitor CT
by the operation of contacts C.sub.6 and connects resistor R14
across capacitor CT, causing it to discharge and opens the RC lead
at contact C.sub.3 in order to discontinue local audible signaling.
Owing to the base current through resistors R15 and R4, transistor
Q1 turns on immediately, causing transistors Q3, Q4, Q5 and Q6 to
turn off, thus releasing relay B. Relay C by way of contact C.sub.2
also disconnects the ringup bridge from the line to remove the
shunting impedance of resistor R2 and capacitor C1 from the
transmission circuit. Relays A and C operated maintain a talking
path by way of contacts A.sub.2 and C.sub.1 and through contacts
C.sub.4 and A.sub.1 connect the L lead to the power supply P3 which
may be on the order of plus or minus ten volts.
The procedure for making an outgoing call is the same as that for
answering an incoming call, except that the transistors Q3, Q4, Q5
and Q6 are normally off and relay B is unoperated.
HOLDING
A busy line can be placed on hold by operating the hold key on
telephone set 103. When the hold key is depressed, ground is
disconnected from the A lead causing relay A to release. Relay A
opens the base current path established when the relay is operated
as described above; the base current path through resistors RT1 and
R4 maintains transistor Q1 in the on state. The A.sub.2 contact
that is shunting the L relay primary opens and, since the station
shunt has not yet been disconnected from the line, the L relay
operates on line current in series with the station shunt.
Operation of contacts L.sub.2 causes the base current of transistor
Q1 to be connected through resistor R4 and diode CR10 to the power
supply P1. The voltage at the base of transistor Q1 drops below the
threshold voltage, transistors Q1 and Q2 turn off, and transistors
Q3, Q4, Q5 and Q6 turn on. Transistors Q5 and Q6 will have turned
on approximately 0.5 milliseconds after relay A releases, and a
hold path is momentarily provided for relay C through resistor R3,
break contact B.sub.6 , transistors Q5 and Q6 and diode CR10, and
thence to power supply P1. Finally, relay B operates through
transistors Q5 and Q6. Relay B operated transfers resistor R3 to
diode CR10 to provide hold current for relay C. Relays B and C
operated connect resistor R1 in series with the primary of relay L
across the line RT to provide a holding path for the central
office, to connect the LG lead to the ST lead through make contact
B.sub.3 and to connect the L lead to the LW lead by way of make
contact C.sub.4. When the hold key is released, the station shunt
is disconnected from the line. Line current through the primary
winding of relay L and resistor R1 maintains the circuit in the
hold state.
RELEASE OF THE HOLDING BRIDGE
Any station of the key telephone system that seizes the line by
operating the associated pickup key and going off-hook will cause
the A relay to operate and to shunt the primary winding of the L
relay which thereby releases. Relay A operated provides current
through resistor R15 by way of make contact A.sub.3 to turn on
transistors Q1 and Q2 immediately. Transistors Q3, Q4, Q5 and Q6
turn off, releasing relay B. Relay C is held by operation of the
A.sub.4 make contacts, and the circuit is thus returned to a busy
state.
When in the hold state, the circuit will bridge line current open
for some preselected period. The circuit will not release from the
hold state, however, when the line current is reversed. The
duration of an absence of line current that is required to release
the circuit from the hold state may be selected through various
wiring options, not shown. Interruption of the line current causes
relay L to release. When relay L releases, the negative end of
charged capacitor C2 is connected to resistor RT1 and through
resistor R4 to the base of transistor Q1, holding Q1 off. During
the line current open interval, capacitor C2 discharges to ground
through resistors RT1 and RT2, driving the base of transistor Q1 in
the direction to turn it on. If the L relay operates before
transistor Q1 turns on, capacitors C2 and C4 recharge, and the
circuit remains in the hold state. If transistors Q1 and Q2 turn on
before the L relay operates, transistors Q3, Q4, Q5 and Q6 turn off
and relay B releases. Relay B releasing, restores the circuit to
its idle state.
DISCONNECTION
When all stations go on-hook, the A lead is disconnected from
ground, causing relay A to release. Release of relay A opens the
holding path for relay C which, in turn, releases. In this way the
circuit is restored to its idle state.
OPERATION WITH LOCAL POWER FAILURE
As indicated above, a local power supply P1 is employed in lieu of
the alternative practice, also conventional, that calls for
deriving all power from the line. In accordance with the invention,
transistor Q7, connected in emitter-follower configuration,
operates as a combined current limiter and constant voltage supply.
The constant voltage at the emitter of transistor Q7 is, in the
embodiment shown, maintained at approximately 17 volts which biases
all of the internal circuitry Q1 through Q4. The emitter of
transistor Q7 is extremely stable which ensures accurate time
constants.
During periods when the local dc supply is non-operative, it is
still possible to originate outgoing calls. When the station goes
off-hook, connection to the line is direct. The primary and
secondary windings of the L relay are connected in series with
resistor R2 and capacitor C1 across the line, but this path has a
negligible effect on the talk circuit. Incoming calls are signaled
by line ringers in the usual way, although visual and common
audible signals are inoperative.
It is to be understood that the embodiment described herein is
merely illustrative of the principles of the invention. Various
modifications thereto may be effected by persons skilled in the art
without departing from the spirit and scope of the invention.
* * * * *